Method op treating magnesia and electrical insulating



P" 1942- R. R. RIDGWAY' EIAL 2,280,516

METHOD OF TREATING. MAGNESIA AND ELECTRlCAL INSULATING MATERIAL DERIVEDTHEREFROM Filed Oct. 27, 1939 3nvento: and Raymond R.R'Ldgwag BBAYch1baIdH.Ba1lard witness (Ittomeg Havbexc E. Covey' Patented Apr. 21,1942 METHOD OF TREATING MAGNESIA AND ELECTRICAL INSULATING MATERIAL DE-RIVED THEREFROM Raymond R. Ridgway, Niagara Falls, N. Y., and ArchibaldH. Ballard, Niagara Falls, Ontario, Canada, assignors to Norton Company,Worcester, Mass, a corporation of Massachusetts Application October 27,1939,-Serial No. 301,643

10 Claims.

This invention relates to a method of improving the electricalinsulating properties of commercially available magnesia and to aninsulating material produced thereby.

Owing to its highly refractive nature and its property of being anon-conductor at temperatures in excess of 1000 C., crystallizedmagnesia, or periclase,,is well adapted for use as an electricalinsulation in high temperature electrical apparatus. This material isparticularly useful in electrical resistance units comprising-aresistance wire embedded in and'separated from a metal sheath by themagnesia, such as is used in electric stoves. Owing to the requirementthat the insulating material possess a high specific resistance so as tominimize the thickness of the insulation and the attendant difference intemperature between the resistor wire and sheath, various attempts havebeen made to improve its electrical resistivity, such as by heating thematerial to a high temperature and slowly cooling it. Nevertheless, ithas been found that the heating units deteriorate with use due to a gradual loss of insulating properties of the magnesia.

The commercial crystalline magnesia obtained from the available orescontains various im-v purities including iron oxide, lime and silica. Wehave discovered that this decrease of resistivity is due to the presenceof iron oxide, and that an amount in excess of 0.2% by weight of ironoxide in magnesia causes a marked and rapid deterioration of the unitduring use. We believe that this is due to the reduction of an externalphase of iron oxide to the metallic and conductive form by the hydrogendeveloped by electrolysis of abnesia which has such physical andchemical characteristics that its resistivity will not deteriorate to amaterial and serious extent before it has given a long life of usefulservice.

A further object of the invention is to provide a method Pf makingcrystalline magnesia from h commercially available materials and sotreating the same that contaminating impurities will not be highly,detrimental and pariicularly so that the iron oxide will be renderedsubstantially innocuous when the magnesia is used as an electricalinsulation. Further objects will be apparent in the followingdisclosure.

In accordance with this invention we have provided an electricalinsulating material for various types of electric heating units. Theaccompanying drawing is a view, partly broken away, of one form ofheating unit embodying this invention. The structure comprises ahelically coiled metal resistance wire I0 located centrally within ametal sheath or casing l l and supported by the special insulatingmaterial l2 as herein described. An electric terminal I3 may be suitablysecured to the resistance wire at each end thereof and similarlyinsulated from the casing by the material I2. Any suitable structuremay, however, be employed.

We have discovered that if the impure magnesia is roasted underoxidizing conditions to insure that the iron content is present asferric oxide, and if the material is allowed to cool slowly in mufilesor kilns then the final product will show a more or less yellow to browncoloration which is related to the oxidized iron content of v theproduct.

this product shows that the magnesia is coated Careful microscopicinspection of with tiny crystals of iron oxide which occur along thefissures and is distributed over the surfaces of themagnesia grains.These almost cryptocrystalline iron oxide crystals, therefore, exist asa free phase which is largely external, and it is this iron-containingphase characteristically distributed over the surfaces of the magnesiacrystals which imparts a uniform coloration to the product. The extentof the brown coloration which shows in a grain containing definite butsmall amounts of iron is proportional to the slowness with which theproduct has cooled from the high temperature.

We found that ifthis product was cooled rapidly, this brown color whollydisappeared or the material became lighter in color, and that the higherthe temperature to which the grain was heated and the quicker it wascooled from this high temperature, the lighter became the color of theresultant refractory powder of magnesia. If a product containing as muchas 0.5% by weight of iron oxide was heated to 1400 C. for 24 hours andthen suddenly quenched, the resultant grain appeared to be free fromiron as judged from the coloring, since it was practically white. Achemical and microscopic investigation showed that the material whichhad been quenched from 1400? C. contained all of the original ferricoxide, but that this oxide was now absorbed into and uniformlydistributed throughout the magnesia crystals. There were no longer thecharacteristic well developed crystals of a phase rich in iron oxideappearing on the surface and throughout the grain boundaries We haveassumed, without proof, that by holding the magnesia grain at a hightemperature for a long time we have caused the ferric oxide to combinewith the magnesia crystals in some form. A possible explanation is thata spinel of the composition of MgO.FezO3 is formed by the prolongedheating, and that this spinel is isomorphous with the magnesia crystalsand so enters into solid solution therewith and results in a uniformdistribution of iron throughout the crystalline mag. nesia phase.

When these two types of material, the slowly cooled and the quenchedmagnesia, were subjected to electrical resistance tests, we found'thatthis difference in physical distribution of the iron resulted inaltogether different resistance characteristics. That is, the heated andquenched material showed a higher initial resistance than the other.Also, this procedure greatly retarded the rate at which such iron richproducts lost their insulating qualities and especially in the presenceof absorbed moisture.

Hence,- it is not necessary to purify the magnesia to the extent ofeliminating all of its iron content, since the insulating properties ofthe material may be greatly enhanced and the useful life thereofprolonged by so treating the material as to insure that the iron contentis present in the oxidized condition but is absorbed or dissolved in ordispersed throughout the ma nesia crystal and does not exist as a freephase external of the individual magnesia crystals where it may bereduced to iron or otherwise develop conducting paths between theresistance wire and the surrounding metallic sheath of a heating unit.Thus, we may use a lower grade of magnesia, i. e., which has a higheriron content, and still produce a product having the characteristics ofa substantially pure magnesia.

In accordance with this discovery We, therefore,

so treat crystallined magnesia as to insure that the iron contentthereof is in the oxide form, and preferably ferric oxide, and We heatthe material to a proper temperature, such as from 1200 to 1400 0., andfor a suitable length of time, preferl ably in excess of 24 hours, andordinarily from 24 to 72 hours, so as to insure that the iron oxidebecomes gradually absorbed into the magnesia crystal grain. This termabsorbedis used broadly to define the location and condition of the ironoxide, without reference to any particular theory as to what happensduring the heating stage. When this condition has been attained, thenthe product is cooled rapidly from its high temperature to acomparatively low temperature, so as to insure that the iron oxide willnot sepa-. rate out as a separate, distinct and external crystallinephase but will remain absorbed within the magnesia crystal, as bycombination with the magnesia or by a physical dispersion throughout themagnesia. During this heating stage, the fine sizes of magnesia arerecrystallized with the development of larger crystals, and theelectrical properties of the material are improved.

This rapid cooling or quenching operation may be accomplished by dumpingthe heated material into a bath of cold water; or the quenching may beaccomplished by quickly spreading out the heated material as a shallowlayer on the floor or in a large metal pan where it is exposed to theelectric resistance unit, and so it cannot develop a metallic phasewhich would readily conduct the current and short circuit the resistanceelement with the grounded metal sheath that is normally used to coverthe same,

Various sources of raw material may be uti-- lized in the preparation ofthis insulating magnesia, and the percentages and nature of the variousimpurities may vary widely. It, however, is desirable to select amaterial which is not highly contaminated and which preferably containsat least 95% of magnesia and not more than 0.5% of iron oxide, andpreferably less than 0.2%. to 3% of silica and from 0 to 2% of calciumoxide. A magnesite ore having these characteristics is readilyavailable. However, an orehaving a higher iron content will bebeneficiated by this procedure as will be readily apparent.

The selected magnesite ore may be calcined to remove CO2 and then fusedin an open top Higgins type are furnace having graphite elec-- trodesand under temperature conditions which result in the complete fusion ofthe material, after which it may be cooled and crystallized. The powerinput may be so controlled as to obtain a temperature in excess of 2600C. which results in the bath being highly fluid and at which the irontends to boil off, and much of it is thus removed from the bath.Standard or desired procedure may be adopted for this furnacingoperation. After the material has beenpartially cooled and solidified inthe furnace, the shell of the furnace is removed and the ingot isallowed to stand on the floor and further cool and crystallize. Theresultant product is crystalline magnesia of the general type of themineral periclase. Since the present-procedure renders the iron oxideharmless, it is feasible to utilize substantially all or the majorportion of the magnesia ingot for the production of electricalinsulation material. Accordingly, the cooled and crystallized ingot isbroken up and crushed in a crusher which is preferably of the type thatwill not contaminate the mass seriously with iron. The material may becrushed to a suitable powdered size, such as one which will pass througha screen of 40 meshes and ,be retained on a screen of 325 meshes perlinear inch, The finer the powder, the more complete will be theseparationpf magnetic conless. The roasting operation is preferablycarried on at a temperature above 1250" C. for at least 48 hours, Duringthis roasting process, air

is admitted freely and oxidizing conditions are The material may alsocontain from 0 maintained so that any metallic iron or lower oxide ofiron that may be present is thoroughly oxidized to ferric oxide. If thegrain is heated in saggers, care should be taken to avoid the presenceof reducing agents which might hinder or prevent the oxidizing action,Hence a reduc ing flame should not contact with the material, but a kilnheated internally with an oxidizing gas or oil flame which provides freeaccess of air is well adapted for the process. The kiln may be linedwith magnesia bricks for the protection of the product. The material isto be held at the required high temperature for a sufficient time,ordinarily over 24 hours, to insure that the iron becomes fully oxidizedand then absorbed in or otherwise distributed within the magnesiacrystal, whether as a solution or combined with the magnesia. The lengthof this soaking period depends upon the temperature maintained and thefinal results desired.

After the material has been sumciently heated, it is quenched so as tocause a quick cooling of the'crystal and prevent the iron oxide fromseparating out as a phase which is external of the magnesia crystal.This may be done, as above explained, by dumping the heated productdirectly from the rotary kiln onto a stone floor and quickly spreadingthe same into a thin layer where it is exposed fully to the externalatmosphere. It is preferable to dump the material into a vat or pancarrying a suflicient quantity of cooling water to cause a rapidlowering of temperature and a stabilization of the iron oxide in itsabsorbed condition. It is of primary importance that the iron oxideremain absorbed in solid solution or be otherwise distributed in themagnesia crystal. It is easy to control the process of quenching, sincethe best results are attained when the product is in its lightestcolored condition. Too slow a cooling gives a tan or light brown colorwhich is an indication of the formation of the objectionable free ironoxide phase to even a slight extent.

The material may contain appreciable amounts of lime and silica, butpreferably within the limits above defined, and without any noticeabledetrimental effects because of such impurities being present. Hence, inaccordance with this invention, a magnesia which contains acomparatively high content of iron, but preferably below 0.5% by weight,and which contains lime and silica may be used satisfactorily as anelectrical insulation material.

For certain purposes, we prefer that the magnesia have a very lowcontent of iron oxide and we may, therefore, use the process which isset forth and claimed in our copending application Serial No. 301,642filed on even date herewith. In accordance with that procedure, wepreferably select a magnesite ore which contains less than 0.2% of ironin the form of iron oxide and we so treat the material throughout itsfusion and other procedure as to reduce that iron content as much aspossible and preferably to make sure that the final product containsless than 0.1% by weight of iron oxide. We also prefer that this ironoxide be present in the highly oxidized condition of ferric oxide orthat it be not readily reducible by hydrogen in the metallic form underthe conditionsof usage, as above indicated. Hence, the carefullyselected magnesite ore is sohandled and treated as to avoid furthercontamination with iron. It is fused in accordance with the procedureabove specified, and in that case we preferably select only that amountof iron oxide.

portion of the ingot which contains the lesser The other part of theingot is discarded because it is found to contain more iron than theintermediate portion, while the extreme center or pipe of the ingotshould also be discarded because of the tendency for the lower meltingimpurities to concentrate in this zone which is the last to cool. Thisselected material is crushed in a way to avoid contamination with ironand then passed through a magnetic separator as above defined toeliminate further iron content. Owing to the boiling ofi of metalliciron in the electric melting operation and the oxidatiomof the iron tothe higher oxide, the iron content is not only very low but it ispresent only in the absorbed condition within the magnesia crystal andthe product has substantially no detrimental external phase of ironoxide.

The following table shows the effect on the electrical resistance of thematerial as caused by the quenching operation:

commercial electrical grade of periclase, or crystalline magnesia,obtainable on the market and which has not been treated in accordancewith this invention. The first column gives the values of the resistancein megohms per inch cube obtained in a standardized method of testingspecific resistance. The second column, Example B, gives the megohms ofthe resistance of material A when employed in a standard commercialelectric stove heating element. The materials of Example C, in column 3were different grades of purified crystalline magnesia, or periclase,which had been purified to reduce the iron content to less than 0.2% byweight, and the measurements of the resistance were made by the sametype of apparatus as that required for Example A. As shown in column 1,the specific resistance of the standard commercial material is nearlydoubled by the quenching operation. The second column, Example B, showsthat the resistance of the material assembled in the heat ing unit ismore than doubled by the quenching step. The materials of Example C hadhigher resistances in the untreated form than had the magnesia ofExample A, because of the lesser iron content. It will, however, benoted that these better grades of magnesia were also materially improvedby the quenching operation. It was also found in a further test that thestove heating element of Example B, when subjectedto a destructive lifetest, failed in approximately 1300 hours; while a stove element havingthe improved insulating powder of Example C has lasted for more than3500 hours at the same temperature.

The benefits of this process are therefore ap- In the above table,Example A is a standard 0 parent, and particularly since the cost ofproduction is lowered and the technical diificulties of producing such ametallurgical product on a large scale are materially lessened by ourbeneficiation of either a low grade or a purified periclase by the abovemethod. The rate at which the electrical resistivity of the magnesiadrops is materially reduced with a great increase in utility and life ofservice of a heating un't or other apparatus containing an insulation ofmagnesia subjected to this heating and quenching procedure. Owing to itsbeing a good conductor of heat and a poor conductor of electricity atthe high temperatures of 850 C. or more, it is well adapted for use instove units and the like at higher temperatures than heretofore foundfeasible. The magnesia conducts radiant heat readily, and since theinsulation does not deteriorate rapidly, as heretofore, it may be usedin a very thin layer, such as 1 to inch, around a standard resistancewire,

which is much thinner than has been the past practice. This reduces thedifference in temperature between the wire and the surrounding metalsheathing and so makes it possible to operate a given heating unit at ahigher temperature than is the case where the untreated magnesia isused. Such a unit formerly limited to about 850 C. may now be used at atemperature as high as 1000 C. with good results.

While we have attempted to explain the phenomena attending thisinvention in the light of our present knowledge, it is to be understoodthat the claims are not to be considered as limited to any particulartheories and that the terms thereof are to be interpreted broadly ascovering the general process above set forth andthe product producedthereby. as well as such equivalent steps and compositions as will nowbe apparent to one skilled in the art in the light of the abovedisclosure. Also, since many variations may be made in the methods ofprocedure and in the composition of the final product, the abovedisclosure is to be considered as illustrating the general principlesand certain preferred features of the invention and not as limitationsthereon, except as the invention is defined by the claims appendedhereto.

We claim:

1. The method of making an electrical insulating material fromcrystalline magnesia contaminated with iron oxide comprising the stepsof heating the crystalline material to a temperature above 1200 C. andforming a product having its iron content present substantially whollyas an oxide absorbed within the magnesia crystals and then quicklycooling the mass and thereby preventing the formation of a materialamount of a separate phase of iron oxide crystals external of themagnesia crystals.

2. The method of making an insulating material from crystalline magnesiacontaminated with iron oxide comprising the steps of heating thematerial under oxidizing conditions to a temperature above 1200 C. andforming a product having its iron content present largely as ferricoxide absorbed within the magnesia crystals and then quickly cooling thesame and preventing the formation of a material amount of a separateexternal phase of iron oxide crystals.

3. The method of making an electrical insulating material comprising thesteps of selecting a material containing at least 95%, by weight ofcrystalline magnesia, from 0 to 3% of silica,

from 0 to 2% of CaO and not over 0.5% by weight of iron oxide, heatingthe material in a crystalline granular condition under oxidizingconditions to a temperature above 1200 C. and forming a product havingits iron oxide content largely absorbed within the magnesia crystals andthereafter cooling the mass rapidly and preventing the iron oxide fromcrystallizing materially as a separate external phase.

4. The method of making an electrical lnsu lating material from animpure magnesia comprising the steps of selecting a grade of magnesiacontaining not over 0.5% by weight of iron oxide, melting andcrystallizing the mass, crushing the crystalline material to a powderwhich will pass through a screen having 40 meshes to the linear inch,heating the crystalline material to a temperature between 1200 and 1400C. for a period of 24 to 72 hours and then quenching the heated materialat a rapid rate at which iron oxide will not crystallize materially as aseparate phase external of the magnesia crystals.

5. The method of making an electrical insulating material comprising thestep of providing crystalline magnesia containing not over 0.2% byweight of iron oxide, not over 3% of silica and not over 2% of calciumoxide, heating the material for several hours under oxidizing conditionsto a temperature as high as 1250 C. and forming a product having theiron content present as ferric oxide substantially wholly absorbedWithin the magnesia crystals, and thereafter cooling the mass rapidlyand preventing the iron oxide from separating out materially as aseparate external crystalline phase.

6. A refractory, heat conductive, electrical resistance heating elementembedding and insulating material comprising primarily granularcrystalline magnesia containing an iron compound in a substantial amountof not over 0.5%

by weight, calculated as ferric oxide, wherein anyiron compound thereinis present substantially wholly as an internal phase within the magnesiacrystals.

7. A refractory, heat conductiv electrical resistance heating elementembedding and insulating material composed chiefly of partiallyrecrystallized, granular, crystalline magnesia containing appreciableamounts of not over 3% by weight of SiOz and not over 2% of CaO and fromabout 0.1% to 0.5% of an iron compound, calculated as Fezos, the totaliron content being present substantially wholly as an internal phase ofan iron compound absorbed within the magnesia crystals.

8. A refractory, heat conductive, electrical heating element embeddingand electrically insulating granular material comprising as its majoringredient crystalline magnesia containing a substantial amount of notover 0.5% by weight of an iron compound, calculated as FezOs, andwherein the total iron content is present substantially wholly as aninternal phase of an iron compound in the highest state of oxidationabsorbed within the magnesia crystals.

9. A granular, refractory, heat conductive, electrical resistanceheating element embedding and electrically insulating material whichcomprises primarily crystalline magnesia containing substantial amountsof not over 3% by weight of SiO2 and not over 2% of CaO and containingfrom about 0.1 to 0.5% of iron compounds, calculated as F6203, the totaliron content existing substantially wholly as an internal phase formedof an iron compound in the highest state of oxidation absorbed withinthe magnesia crystals.

10. A granular, refractory, heat conductive, of from about 0.1 to 0.5%by weight, calculated electrical resistance heating element embedding asi'erric oxide, the iron content existing suband electrically insulatingmaterial which is comstantially wholly as an internal phase of a ferricposed chiefly of partially recrystallized, granucompound absorbed withinthe magnesia cryslar, crystalline magnesia containing substantial 5tals. amounts of not over 3% of B10: and not over RAYMOND R. RIDGWAY. 2%oi CaO and containing a total iron content ARCHIBAID H. BALLARD.

